A conventional method of cleaning silicon substrates to be used as material of semiconductors has used hydrogen fluoride, mixed solution such as sulfuric acid and hydrogen peroxide water, mixed solution of hydrochloric acid and hydrogen peroxide water, or mixed solution of ammonia water and hydrogen peroxide water. A silicon substrate is dipped into a bath in which the foregoing mixed solution is filled. For instance, after boron implantation, the silicon substrate undergoes an anneal process for diffusing the boron. However, before undergoing the anneal process, the silicon substrate must be cleaned with a solution diluted at a given concentration. The solution includes hydrogen fluoride, mixed solution of sulfuric acid and hydrogen peroxide water, mixed solution of hydrochloric acid and hydrogen peroxide water, or mixed solution of ammonia water and hydrogen peroxide water. Bathes containing one of the foregoing solutions and the bath containing extra-pure water are arranged, and the solutions are combined or a cleaning time is adjusted depending on an application of the substrate, so that the substrate is cleaned.
Ion implantation is used as a method for introducing particles into silicon substrates, and plasma-doping is a promising method to be used in the next generation for introducing particles into substrates efficiently with ultra-low energy. After the introduction of particles, a level of metal contamination is a cause for concern, in particular, when the plasma doping is used because of the following reason: In the case of ion implantation, particles other than desired ones are removed by an analytical electromagnet; however, the plasma doping does not use the analytical electromagnet and exposes the silicon substrate directly to plasma. In other words, the chamber inner wall is exposed to plasma, and the silicon substrate is exposed to the same plasma, so that metal contamination caused by the chamber tends to occur. Such a study is reported in, e.g. the following document: “Proc. Of the 1997 IEEE International Symposium on Semiconductor Manufacturing Conference, San Francisco, Calif., Oct. 6-8, 1997, p. B-9”, and “Proc. of the 2000 International Conference on Ion Implantation Technology, Alpbach, Austria, 17-22 Sep. 2000, p. 500-503”
According to the foregoing reports, the concerns about the metal contamination can be solved at a research and development stage; however, the contamination level is not yet improved as good as that of the ion implantation.
In general, not only the plasma doping but also the ion implantation aggravates the level of contamination, e.g. metal contamination of an object such as a silicon substrate after the introduction of particles comparing with the level before the introduction thereof, and the contamination is caused by an impurity introducing apparatus. The metal contamination is one of the factors lowering an yield rate of semiconductor devices. Hydrogen fluoride has higher cleaning power than the mixed solution of sulfuric acid and hydrogen peroxide water, so that solution including hydrogen fluoride is often used for cleaning silicon substrates after the ion implantation.
After introduction of particles into a silicon substrate, and before an anneal process, a cleaning of the silicon substrate reveals another problem in the case when the particles are introduced with so little energy that the depth becomes not greater than 15 nm, more particularly not greater than 10 nm, which makes an atomic concentration of the particles 1E18 atoms/cm3. To be more specific, particles introduced in the substrate at such a shallow depth are washed away in the cleaning step, although the particles are introduced to be dopant. Thus fewer particles are electrically activated at the anneal process, and as a result, desirable characteristics cannot be expected.
To overcome the foregoing problem is important for the plasma doping among others. The plasma doping introduces particles of dopant not only by doping charged particles through acceleration into an object such as a silicon substrate, but also through adsorption and infiltration of particles by using radical and gas onto a principal surface of the silicon substrate. The particles are introduced by radical and gas with so low energy that the particles are introduced at a shallow depth of the silicon substrate. This is different from the behavior of the charged particles which are doped by an accelerated voltage. This mechanism inevitably allows introducing particles with low energy by radical and gas although an accelerating voltage of several hundreds volt is applied to plasma by controlling a bias electric potential. The particles introduced with such low energy out of all the dopant will be washed away, so that a desirable sheet resistance cannot be expected as discussed above.
It is essential to clean semiconductor substrates including silicon substrates for maintaining yields of semiconductor devices. Concerning the plasma doping among others, it is not yet reported that its contamination level reaches to a level similar to that of the ion implantation in manufacturing, so that a method of cleaning after the doping becomes critical.
A cleaning method of semiconductor substrates is needed because of the foregoing circumstances. The required method preferably cleans semi-conductor substrates before the anneal process at a level maintaining yields of semi-conductor devices, and yet, maintains an amount of introduced particles throughout the cleaning step.
On the other hand, the following method is widely used: a resist-pattern is formed on, e.g. a silicon substrate, and impurity is introduced onto an exposed section of a principal surface of the silicon substrate. The resist is attached onto the substrate in a desired pattern before the impurity is introduced, and removed after the introduction but before the anneal process. The resist is often removed by an ashing method, which irradiates the resist with oxygen-plasma. When particles are introduced at a shallow depth of the principal surface, removal of resist by a conventional method before the anneal process will remove particles together with the resist although the particles have been introduced to be dopant. Thus the anneal process activates electrically fewer particles, so that desirable characteristics cannot be expected.
The foregoing situation needs a better method of removing resist or an appropriate combination of a resist-removing method and an impurity-introducing method. The better method or the appropriate combination preferably removes resist attached to the substrate after the introduction of particles but before an anneal process, and yet, preferably maintains an amount of the introduced particles throughout the resist-removing step.